By virtue of recent progress of low-dose photography, a radiation dose in full scan used for diagnosis is increasingly lowering. In order to determine a scan position and range and a reconstruction position and range of the full scan and calculate an optimum radiation dose in a scan planning stage, two-dimensional positioning images (also called positioning image scanograms) are photographed. The positioning image is photographed by repeating photographing continuously while moving a top board at a constant speed or intermittently or intermittently in sync with the movement of the top board with an X-ray tube fixed at a 0° position, that is, the front position with respect to a subject, for example. Although the positioning image may be collected from, in addition to the front direction, two side directions and further certain directions, a display direction is fixed to the photographing position, and the display direction cannot be changed after photographing.
An exposure dose required for photographing the positioning image has not been much changed in spite of a recent reduction in an exposure dose of the full scan. There is a problem in that automatic exposure control used in CT scan (CT-AEC), if the height of a bed changes, changes calculated mA even for the same subject due to a magnification. The positioning image is merely a projection image (also called a projection image) and is not much favorable in the setting accuracy and the convenience of setting operation of a scan range or the like in scan planning.